1. Field of the Invention
The invention relates to production, processing, and use of thermoplastics that are dimensionally stable under heats and in particular, to molding compositions based on imidated polymethyl methacrylate. It describes molding compositions that, after processing to molding articles, for example, lamp shades, exhibit only a slight increase in yellowness.
2. Discussion of the Background
Polymers based on imidated polymethyl methacrylate are a unique class of highly transparent and thereby particularly dimensionally stable thermoplastics when subjected to heat. Molded articles made of this material can be exposed, in the long run, to temperatures clearly higher than molded articles made of other highly transparent thermoplastics such as polymethyl methacrylate (PMMA). Of course, the danger of discoloration also increases with the higher thermal stress In order to be able to use these high-performance thermoplastics, for example, as shades for lamps, it is necessary to protect them, as much as possible, from thermally induced discoloration, which is visible as an increase in yellowness. Yellowness is measured in terms of the yellowness value according to DIN 6167 (D65/10) or according to ASTM D 1925.
RD 321,114 describes a method for reducing the yellowish tinge of polymethacrylimides, by carrying out imidation in an oxygen-free atmosphere. The removal of oxygen can be achieved only by an expensive distillation process, which is unfavorable economically.
European Patent No. A 576 877 describes a polymer based on polymethacrylimide and polyacrylimide with a low yellowness value, wherein salts of phosphine or phosphonic acid are added during the imidation reaction. The imidation reaction consists of reaction of a polymer based on C.sub.1 -C.sub.20 akyl esters of methacrylic acid and/or acrylic acid with ammonia or a primary-alkyl-substituted amine. It is carried out in a malt or in a solution under high pressure and at high temperature. According to European Patent No. A 576 877, the phosphorus compound is added to the reaction mixture and then exposed to these drastic conditions. The synthesis results in a molding composition that exhibits a relatively low yellow coloring. Upon consideration of this disclosure by the inventors, it was discovered that the molded articles produced from these molding compositions exhibit clear yellowing under thermal stress. Only at the beginning of molding is yellowing at a low level. As the thermal stress continues, however, the yellowness value steadily rises. The presence of the previously added reducing phosphorus compounds can hardly impart any stability against yellowing. It must, therefore, be assumed that under the imidation conditions, the phosphorus compound has been extensively consumed or it has been decomposed.
The quantities of phosphorus compounds employed in such compositions are accordingly high, apparently in order to compensate for a loss in effectiveness of the compounds. Quantities of 0.1-1wt %, based on the quantity of polymers to be imidated, are preferably used. However, an increase in amounts of the phosphorus compound incorporated in the molding composition is not feasible, because other characteristics of the polymer are consequently impaired. According to what is known to the inventors, turbidity appears in the polymer with the addition of more than 0.1 wt % of the reducing phosphorus compound in the molding composition.
In experiments conducted by the inventors in accordance with European Patent No. A 576 877, it was found that phosphine was generated, in particular at high compound concentrations in the molding compositions. The generation of phosphine may, therefore, be used as an index of the decomposition of the reducing phosphorus compound in the method of the publication. Disproportionation of the hypophosphite additive should, therefore, be assumed to be one of the decomposition reactions which take place.
The use of reducing organic phosphorus compounds as antioxidants, which, among other things, are supposed to prevent the discoloration of plastic molding compositions with thermal stress, is known (see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 3, p. 133, Wiley, New York, 1978). From case to case, they are also added to molding compositions before processing, that is, during the compounding step. Thus, in accordance with Japanese Application Kokai Tokkyo Koho JP 60 123 547, an improvement of the discoloration of copolymers of methyl methacrylate, styrene, and maleic anhydride monomer units, under injection molding conditions and at higher temperatures, is observed, if such copolymers are mixed with at least one phosphaphenanthrene derivative and also a sterically hindered phenol, a thiopropionic acid ester or a phosphorus acid ester as stabilizers against oxidative degradation, before processing by injection molding. Japanese Kokai Tokkyo Koho JP 60 120 735 describes copolymers of methyl methacrylate, vinyl aromatic substance, and polymerized-in cyclic anhydrides, to which phosphoric acid esters and other stabilizers based on sterically hindered phenols are added to increase thermal stability and to prevent the discoloration of such copolymers under thermal stress in the melt, for example, in injection molding.
Japanese Kokai Tokkyo Koho JP 03 167 245 claims the stabilization of copolymers of methyl methacrylate, N-substituted maleinimides and other copolymerizable monomers with compounds selected from the group of alkyl-substituted triaryl phosphates, the dialkyl pentaerythritol diphosphites, and the phosphaphenanthrene derivatives.
Japanese Kokai Tokkyo Koho JP 63 163 306 discloses copolymers of methyl methacrylate and C.sub.8 -C.sub.20 alkyl methacrylate as the core material for optical light-conducting fibers, which contain, as stabilizers, phosphites, such as sterically hindered diaryl pentaerythritol diphosphites, or thiophosphites to prevent the discoloration of the copolymers under thermal stress.
In the four Japanese patents mentioned here, sterically hindered, organic phosphites are mentioned without exception, or organic phosphites, together with sterically hindered phenols. Inorganic reducing phosphorus compounds are not mentioned.
Also, German Utility Model No. 29,504,693.7 describes the use of sterically hindered organic phosphate compounds in molding compositions of copolymers, which consist of the monomer units of alkyl methacrylate, vinyl aromatic, and maleic anhydride, and optionally alkyl acrylate. The organic phosphorus compounds are added, from case to case, also to the finished, granulated, or comminuted polymer before further processing.
Organic phosphorus compounds, namely sterically hindered organophosphites or organophosphonites, are also used, according to European Patent No. A 396 336, to prevent discoloration of polymers based on (N-alkyl)dialkylglutarimide and (N-hydrogen)dialkylglutarimide and to stabilize the polymers against bubble formation. European Patent No. A 463 754 discloses the use of trialkyl phosphates and aliphatic dicyclodiphosphites for the same purpose.
In the last three patent applications mentioned above, only organic reducing statically hindered phosphorus compounds are also mentioned, but not inorganic reducing phosphorus compounds. A need, therefore, continues to exist for imidated polymethyl methacrylate molding compositions which exhibit improved stability against yellowing.
The known art techniques do not permit one of skill to expect the successful preparation of any molding composition, based on imidated polymethyl methacrylate, which is suitable for the production of highly transparent, yellow free molded articles, which is dimensionally stable with heat and which also exhibits a reduced yellowness value increase even when subjected to continuous thermal stress.